Some papermaking processes require the drying of the paper web being formed to under three per cent (3%) moisture content at one or more points in the process. The manufacture of paper and paperboard which must be dried to less than 3% moisture content has been accompanied by chronic problems which have adversely affected the efficiency and increased the cost of producing the paper product. Drying a paper or paperboard web to a moisture content of less than 3% requires a high energy input. Heretofore, it was difficult to control drying of a paper or paperboard web to less than 3% moisture in such a way that untoward effects could be avoided, so that the quality of the paper product has not always been consistent or predictable. The present invention effectively controls the drying of a paper web to less than 3% moisture so that energy usage is reduced and the quality of the final product is significantly improved.
During the papermaking drying process a non-uniform drying stress distribution may develop in both the sheet plane direction and in the thickness direction because of non-uniformity in the hydro-thermal and mechanical properties produced in the wire and press sections of the papermaking apparatus and because of the non-uniform moisture and temperature history during drying. Curl, wrinkle, cockle and other results of dimensional instability in the paper drying process are likely to be produced in the finished sheet. It has been recognized that the distribution of drying stress can be altered by exposing the paper web to different drying conditions or "histories" on the top and bottom sides in the after-dryer section of the papermaking process. Although curl shape at the reel can be affected somewhat by this type of differential drying, the basic dimensional instabilities created in a moving paper web by non-uniform drying stress have not been eliminated from the final product. Nari et al recognized the critical nature of and demand for dimensional stability in a wide variety of paper types and investigated the relationship between the hygroexpansion coefficient and drying shrinkage in papers made form mechanical pulps in Tappi Journal, Vol. 76, No. 6 (June 1993), finding that low drying shrinkages for some mechanical pulps were caused by low moisture changes rather than a low hygroexpansion coefficient.
The presence of uneven cross-direction drying rates induces mechanical stresses in the paper sheet during the papermaking process. When the paper sheet is dried under tension this stress is "locked in" as different parts of the sheet shrink and dry at different places, rates and times during the process. This physical stress can be released by rewetting so that a repeatedly rewetted and redried paper sheet with varying cross-direction stress areas is highly likely to display such adverse effects as baggy edges, cockles, soft centers and hard centers which detract significantly from the paper quality. An effective control system for a papermaking drying process would effectively eliminate these problems.
The drying of paper and paperboard consumes large quantities of energy and accounts for much of the cost of the finished product. Accordingly, a great deal of work has focused on reducing the energy consumed in drying by measuring the moisture profile in the web and then modulating the amount of drying energy supplied to various strips of the web to achieve a match between the amount of energy supplied to each longitudinal strip in the web and the moisture content of that strip. These methods obtain a relatively flat moisture profile in the sheet with a reduced expenditure of energy. Even though such methods seem to achieve significant reductions in energy consumption, they typically overdry the web which produces adverse effects in the finished paper product.
The prior art has disclosed a wide variety of systems and methods for drying moving paper webs during production to desired dryness specifications. U.S. Pat. Nos. 4,494,316 to Stephansen et al; and 4,509,270; 4,514,913 and 4,594,795 to Stephansen for example, all disclose systems for drying moving paper webs. These cross-direction web dryers are designed to reduce the moisture content of the web. U.S. Pat. No. 4,514,913 measures the web moisture profile and relocates dryer modules over the worst moisture streaks in the moving web.
U.S. Pat. No. 4,590,685 to Roth discloses an apparatus for uniformly drying a paper web to a desired moisture content which includes a manual or automatic control system responsive to the web moisture content. Spaced parallel drying units are regulated by the control system between low and high flame conditions to resolve narrow moisture streaks in the cross web direction. U.S. Pat. No. 3,293,770 to Rauskolb also discloses a radiant heat web dryer that is adjustable in response to web moisture levels to vary the drying, effects. The sheet material drying system disclosed in U.S. Pat. No. 3,720,002 to Martin uses a combination of radiant heat and heated gas to dry a wet web, depending on the web moisture content.
The use of the web moisture content as a control parameter for optimum paper drying has not proved to be an ideal solution to the aforementioned problems. The available web moisture measuring equipment has often produced misleading or inaccurate information, which has resulted in the overdrying of the web. In papermaking processes requiring low web moisture contents, particularly moisture contents of less than 3%, web overdrying can easily produce a degraded paper product.
The prior art has proposed dryer systems useful in sheet manufacturing processes which use control parameters other than moisture content as a basis for controlling the drying energy applied to the sheet. U.S. Pat. No. 4,612,802 to Clarke et al, for example, discloses a method and apparatus for determining the moisture content of a thin wood sheet by monitoring the variations in the rise and fall of the sheet surface temperature after the sheet has been heated. However, there is no suggestion that this method could be used to control drying in a papermaking process to produce optimal drying when the paper moisture content is less than 3%.
The dryer control system disclosed in U.S. Pat. No. 4,701,857 to Robinson monitors two temperatures, that of the sheet product being dried and that of the drying medium, and determines what the final moisture content will be. The temperature differential or sheet speed is then controlled to obtain the desired final moisture content. This control system, which is more suitable for wood products than for paper sheets, does not use the temperature profile of the sheet itself as a basis for optimizing drying of the sheet to a moisture content of about 3% with a minimum expenditure of energy.
U.S. Pat. No. 5,010,659 to Trelevan discloses an infrared drying system which monitors the moisture content, temperature or other physical property at selected zone positions along the width of a traveling web. Infrared heating lamps are controlled and energized by a computer control system in response to the selected physical property. The moisture content or other physical property of the web is sensed at only one location of the web in this system, and it is not suggested that optimization of drying to a low web moisture content of 3% by obtaining a web temperature profile could be accomplished with this system.
The prior art, therefore, has failed to provide a papermaking process drying control system or method which monitors the web cross-direction temperature profile of the paper web to optimize drying of the web to a low moisture content of about 3% at low energy usage which is not accompanied by the adverse effects in the final product produced by uneven drying. A need exists for such a system and method.